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class="sidebar-link">面试官：请描述下你对vue生命周期的理解？在created和mounted这两个生命周期中请求数据有什么区别呢？</a></li><li><a href="/fe/vue/if_for.html" class="sidebar-link">面试官：v-if和v-for的优先级是什么？</a></li><li><a href="/fe/vue/computed_watch.html" class="sidebar-link">计算属性 VS 监听属性</a></li><li><a href="/fe/vue/first_page_time.html" class="sidebar-link">面试官：SPA首屏加载速度慢的怎么解决？</a></li><li><a href="/fe/vue/data.html" class="sidebar-link">面试官：为什么data属性是一个函数而不是一个对象？</a></li><li><a href="/fe/vue/data_object_add_attrs.html" class="sidebar-link">面试官：动态给vue的data添加一个新的属性时会发生什么？怎样解决？</a></li><li><a href="/fe/vue/components_plugin.html" class="sidebar-link">面试官：Vue中组件和插件有什么区别？</a></li><li><a href="/fe/vue/communication.html" class="sidebar-link">面试官：Vue组件之间的通信方式都有哪些？</a></li><li><a href="/fe/vue/nexttick.html" class="sidebar-link">面试官：Vue中的$nextTick有什么作用？</a></li><li><a href="/fe/vue/mixin.html" class="sidebar-link">Mixin 混入</a></li><li><a href="/fe/vue/slot.html" class="sidebar-link">面试官：说说你对slot的理解？slot使用场景有哪些？</a></li><li><a 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class="sidebar-link">面试官：说下你的vue项目的目录结构，如果是大型项目你该怎么划分结构和划分组件呢？</a></li><li><a href="/fe/vue/permission.html" class="sidebar-link">面试官：vue要做权限管理该怎么做？如果控制到按钮级别的权限怎么做？</a></li><li><a href="/fe/vue/cors.html" class="sidebar-link">面试官：Vue项目中你是如何解决跨域的呢？</a></li><li><a href="/fe/vue/404.html" class="sidebar-link">面试官：vue项目本地开发完成后部署到服务器后报404是什么原因呢？</a></li><li><a href="/fe/vue/error.html" class="sidebar-link">面试官：你是怎么处理vue项目中的错误的？</a></li><li><a href="/fe/vue/vue_router.html" class="sidebar-link">Vue Router</a></li><li><a href="/fe/vue/vue-debounce-throttle.html" class="sidebar-link">vue防抖调研</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue3</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>React</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Webpack</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Node.js</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="mvvm-双向数据绑定"><a href="#mvvm-双向数据绑定" class="header-anchor">#</a> MVVM 双向数据绑定</h1> <p><img src="https://static.vue-js.com/cef7dcc0-3ac9-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h2 id="一、什么是双向绑定"><a href="#一、什么是双向绑定" class="header-anchor">#</a> 一、什么是双向绑定</h2> <p>我们先从单向绑定切入，就是把<code>Model</code>绑定到<code>View</code>，当我们用<code>JavaScript</code>代码更新<code>Model</code>时，<code>View</code>就会自动更新，双向绑定就很容易联想到了，在单向绑定的基础上，用户更新了<code>View</code>，<code>Model</code>的数据也自动被更新了，这种情况就是双向绑定举个栗子</p> <p><img src="https://static.vue-js.com/d65738d0-3ac9-11eb-ab90-d9ae814b240d.png" alt=""></p> <p>当用户填写表单时，<code>View</code>的状态就被更新了，如果此时可以自动更新<code>Model</code>的状态，那就相当于我们把<code>Model</code>和<code>View</code>做了双向绑定关系图如下</p> <p><img src="https://static.vue-js.com/dcc1d4a0-3ac9-11eb-ab90-d9ae814b240d.png" alt=""></p> <blockquote><p>以下是其他解释：</p></blockquote> <p><strong>Model 层</strong>：对应服务端的数据</p> <p><strong>View 层</strong>：视图模板，做的是数据绑定的声明、 指令的声明、 事件绑定的声明</p> <p><strong>ViewModel 层</strong>：View层 需要的数据，负责具体业务逻辑</p> <p>Vue 的 MVVM 双向数据绑定是通过 <strong>数据劫持 + 发布订阅模式</strong> 实现的</p> <p>真正实现其实靠的也是<code>ES5</code>中提供的 <code>Object.defineProperty</code>（定义对象上的属性和值），当然这是不兼容的所以Vue等只支持了<code>IE8+</code></p> <p><strong>优点：</strong></p> <ol><li>自动更新dom: 利用双向绑定,数据更新后视图自动更新,让开发者从繁琐的手动dom中解放</li> <li>分离视图（View）和模型（Model）,降低代码耦合，提高视图或者逻辑的重用性</li> <li>提高可测试性: ViewModel的存在可以帮助开发者更好地编写测试代码</li></ol> <p><strong>缺点：</strong></p> <ol><li>产生性能问题，对于简单的应用会造成额外的性能消耗。</li> <li>对于复杂的应用，视图状态较多，视图状态的维护成本增加，ViewModel构建和维护成本高。</li> <li>Bug很难被调试: 因为使用双向绑定的模式，当你看到界面异常了，有可能是你View的代码有Bug，也可能是Model的代码有问题。</li></ol> <h2 id="二、双向绑定的原理是什么"><a href="#二、双向绑定的原理是什么" class="header-anchor">#</a> 二、双向绑定的原理是什么</h2> <p>我们都知道 <code>Vue</code> 是数据双向绑定的框架，双向绑定由三个重要部分构成</p> <ul><li>数据层（Model）：应用的数据及业务逻辑</li> <li>视图层（View）：应用的展示效果，各类UI组件</li> <li>业务逻辑层（ViewModel）：框架封装的核心，它负责将数据与视图关联起来</li></ul> <p>而上面的这个分层的架构方案，可以用一个专业术语进行称呼：<code>MVVM</code>这里的控制层的核心功能便是 “数据双向绑定” 。自然，我们只需弄懂它是什么，便可以进一步了解数据绑定的原理</p> <div class="language-text extra-class"><pre class="language-text"><code>- ES5: `Object.defineProperty` - 2.0
- ES6: `Proxy` - 3.0

使用Object.defineProperty缺点：

1. 对原始数据克隆
2. 需要分别给对象中的每一个对象属性设置监听
</code></pre></div><h3 id="理解viewmodel"><a href="#理解viewmodel" class="header-anchor">#</a> 理解ViewModel</h3> <p>它的主要职责就是：</p> <ul><li>数据变化后更新视图</li> <li>视图变化后更新数据</li></ul> <p>当然，它还有两个主要部分组成</p> <ul><li>监听器（Observer）：对所有数据的属性进行监听</li> <li>解析器（Compiler）：对每个元素节点的指令进行扫描跟解析,根据指令模板替换数据,以及绑定相应的更新函数</li></ul> <h2 id="三、实现双向绑定"><a href="#三、实现双向绑定" class="header-anchor">#</a> 三、实现双向绑定</h2> <p>我们还是以<code>Vue</code>为例，先来看看<code>Vue</code>中的双向绑定流程是什么的</p> <ol><li><code>new Vue()</code>首先执行初始化，对<code>data</code>执行响应化处理，这个过程发生在<code>Observer</code>中</li> <li>同时对模板执行编译，找到其中动态绑定的数据，从<code>data</code>中获取并初始化视图，这个过程发生在<code>Compile</code>中</li> <li>同时定义⼀个更新函数和<code>Watcher</code>，将来对应数据变化时<code>Watcher</code>会调用更新函数</li> <li>由于<code>data</code>的某个<code>key</code>在⼀个视图中可能出现多次，所以每个<code>key</code>都需要⼀个管家<code>Dep</code>来管理多个<code>Watcher</code></li> <li>将来data中数据⼀旦发生变化，会首先找到对应的<code>Dep</code>，通知所有<code>Watcher</code>执行更新函数</li></ol> <p>流程图如下：</p> <p><img src="https://static.vue-js.com/e5369850-3ac9-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h3 id="实现"><a href="#实现" class="header-anchor">#</a> 实现</h3> <p>先来一个构造函数：执行初始化，对<code>data</code>执行响应化处理</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">class</span> <span class="token class-name">Vue</span> <span class="token punctuation">{</span>  
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">options</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>$options <span class="token operator">=</span> options<span class="token punctuation">;</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>$data <span class="token operator">=</span> options<span class="token punctuation">.</span>data<span class="token punctuation">;</span>  
        
    <span class="token comment">// 对data选项做响应式处理  </span>
    <span class="token function">observe</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>$data<span class="token punctuation">)</span><span class="token punctuation">;</span>  
        
    <span class="token comment">// 代理data到vm上  </span>
    <span class="token function">proxy</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
        
    <span class="token comment">// 执行编译  </span>
    <span class="token keyword">new</span> <span class="token class-name">Compile</span><span class="token punctuation">(</span>options<span class="token punctuation">.</span>el<span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
</code></pre></div><p>对<code>data</code>选项执行响应化具体操作</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">observe</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
  <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> obj <span class="token operator">!==</span> <span class="token string">&quot;object&quot;</span> <span class="token operator">||</span> obj <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">return</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
  <span class="token keyword">new</span> <span class="token class-name">Observer</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">}</span>  
  
<span class="token keyword">class</span> <span class="token class-name">Observer</span> <span class="token punctuation">{</span>  
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">value</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>value <span class="token operator">=</span> value<span class="token punctuation">;</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">walk</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
  <span class="token function">walk</span><span class="token punctuation">(</span><span class="token parameter">obj</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    Object<span class="token punctuation">.</span><span class="token function">keys</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">key</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>  
      <span class="token function">defineReactive</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> key<span class="token punctuation">,</span> obj<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
</code></pre></div><h4 id="编译compile"><a href="#编译compile" class="header-anchor">#</a> 编译<code>Compile</code></h4> <p>对每个元素节点的指令进行扫描跟解析,根据指令模板替换数据,以及绑定相应的更新函数</p> <p><img src="https://static.vue-js.com/f27e19c0-3ac9-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">class</span> <span class="token class-name">Compile</span> <span class="token punctuation">{</span>  
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span> vm</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>$vm <span class="token operator">=</span> vm<span class="token punctuation">;</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>$el <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">querySelector</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 获取dom  </span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token keyword">this</span><span class="token punctuation">.</span>$el</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
      <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">compile</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>$el<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token punctuation">}</span>  
  <span class="token punctuation">}</span>  
  <span class="token function">compile</span><span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">const</span> childNodes <span class="token operator">=</span> el<span class="token punctuation">.</span>childNodes<span class="token punctuation">;</span>   
    Array<span class="token punctuation">.</span><span class="token function">from</span><span class="token punctuation">(</span>childNodes<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">node</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span> <span class="token comment">// 遍历子元素  </span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">isElement</span><span class="token punctuation">(</span>node<span class="token punctuation">)</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>   <span class="token comment">// 判断是否为节点  </span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">&quot;编译元素&quot;</span> <span class="token operator">+</span> node<span class="token punctuation">.</span>nodeName<span class="token punctuation">)</span><span class="token punctuation">;</span>  
      <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">isInterpolation</span><span class="token punctuation">(</span>node<span class="token punctuation">)</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">&quot;编译插值⽂本&quot;</span> <span class="token operator">+</span> node<span class="token punctuation">.</span>textContent<span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 判断是否为插值文本 {{}}  </span>
      <span class="token punctuation">}</span>  
      <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token parameter">node<span class="token punctuation">.</span>childNodes <span class="token operator">&amp;&amp;</span> node<span class="token punctuation">.</span>childNodes<span class="token punctuation">.</span>length <span class="token operator">&gt;</span> <span class="token number">0</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  <span class="token comment">// 判断是否有子元素  </span>
        <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">compile</span><span class="token punctuation">(</span>node<span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// 对子元素进行递归遍历  </span>
      <span class="token punctuation">}</span>  
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
  <span class="token function">isElement</span><span class="token punctuation">(</span><span class="token parameter">node</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">return</span> node<span class="token punctuation">.</span>nodeType <span class="token operator">==</span> <span class="token number">1</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
  <span class="token function">isInterpolation</span><span class="token punctuation">(</span><span class="token parameter">node</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">return</span> node<span class="token punctuation">.</span>nodeType <span class="token operator">==</span> <span class="token number">3</span> <span class="token operator">&amp;&amp;</span> <span class="token regex"><span class="token regex-delimiter">/</span><span class="token regex-source language-regex">\{\{(.*)\}\}</span><span class="token regex-delimiter">/</span></span><span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span>node<span class="token punctuation">.</span>textContent<span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
  
</code></pre></div><h4 id="依赖收集"><a href="#依赖收集" class="header-anchor">#</a> 依赖收集</h4> <p>视图中会用到<code>data</code>中某<code>key</code>，这称为依赖。同⼀个<code>key</code>可能出现多次，每次都需要收集出来用⼀个<code>Watcher</code>来维护它们，此过程称为依赖收集多个<code>Watcher</code>需要⼀个<code>Dep</code>来管理，需要更新时由<code>Dep</code>统⼀通知</p> <p><img src="https://static.vue-js.com/fa191f40-3ac9-11eb-ab90-d9ae814b240d.png" alt=""></p> <p>实现思路</p> <ol><li><code>defineReactive</code>时为每⼀个<code>key</code>创建⼀个<code>Dep</code>实例</li> <li>初始化视图时读取某个<code>key</code>，例如<code>name1</code>，创建⼀个<code>watcher1</code></li> <li>由于触发<code>name1</code>的<code>getter</code>方法，便将<code>watcher1</code>添加到<code>name1</code>对应的Dep中</li> <li>当<code>name1</code>更新，<code>setter</code>触发时，便可通过对应<code>Dep</code>通知其管理所有<code>Watcher</code>更新</li></ol> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// 负责更新视图  </span>
<span class="token keyword">class</span> <span class="token class-name">Watcher</span> <span class="token punctuation">{</span>  
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">vm<span class="token punctuation">,</span> key<span class="token punctuation">,</span> updater</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>vm <span class="token operator">=</span> vm  
    <span class="token keyword">this</span><span class="token punctuation">.</span>key <span class="token operator">=</span> key  
    <span class="token keyword">this</span><span class="token punctuation">.</span>updaterFn <span class="token operator">=</span> updater  
  
    <span class="token comment">// 创建实例时，把当前实例指定到Dep.target静态属性上  </span>
    Dep<span class="token punctuation">.</span>target <span class="token operator">=</span> <span class="token keyword">this</span>  
    <span class="token comment">// 读一下key，触发get  </span>
    vm<span class="token punctuation">[</span>key<span class="token punctuation">]</span>  
    <span class="token comment">// 置空  </span>
    Dep<span class="token punctuation">.</span>target <span class="token operator">=</span> <span class="token keyword">null</span>  
  <span class="token punctuation">}</span>  
  
  <span class="token comment">// 未来执行dom更新函数，由dep调用的  </span>
  <span class="token function">update</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">updaterFn</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>vm<span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">.</span>vm<span class="token punctuation">[</span><span class="token keyword">this</span><span class="token punctuation">.</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
</code></pre></div><p>声明<code>Dep</code></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">class</span> <span class="token class-name">Dep</span> <span class="token punctuation">{</span>  
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>deps <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>  <span class="token comment">// 依赖管理  </span>
  <span class="token punctuation">}</span>  
  <span class="token function">addDep</span><span class="token punctuation">(</span><span class="token parameter">dep</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>deps<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>dep<span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
  <span class="token function">notify</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>   
    <span class="token keyword">this</span><span class="token punctuation">.</span>deps<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">dep</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> dep<span class="token punctuation">.</span><span class="token function">update</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
</code></pre></div><p>创建<code>watcher</code>时触发<code>getter</code></p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">class</span> <span class="token class-name">Watcher</span> <span class="token punctuation">{</span>  
  <span class="token function">constructor</span><span class="token punctuation">(</span><span class="token parameter">vm<span class="token punctuation">,</span> key<span class="token punctuation">,</span> updateFn</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
    Dep<span class="token punctuation">.</span>target <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">;</span>  
    <span class="token keyword">this</span><span class="token punctuation">.</span>vm<span class="token punctuation">[</span><span class="token keyword">this</span><span class="token punctuation">.</span>key<span class="token punctuation">]</span><span class="token punctuation">;</span>  
    Dep<span class="token punctuation">.</span>target <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>  
  
</code></pre></div><p>依赖收集，创建<code>Dep</code>实例</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">defineReactive</span><span class="token punctuation">(</span><span class="token parameter">obj<span class="token punctuation">,</span> key<span class="token punctuation">,</span> val</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
  <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">observe</span><span class="token punctuation">(</span>val<span class="token punctuation">)</span><span class="token punctuation">;</span>  
  <span class="token keyword">const</span> dep <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Dep</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
  Object<span class="token punctuation">.</span><span class="token function">defineProperty</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> key<span class="token punctuation">,</span> <span class="token punctuation">{</span>  
    <span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
      Dep<span class="token punctuation">.</span>target <span class="token operator">&amp;&amp;</span> dep<span class="token punctuation">.</span><span class="token function">addDep</span><span class="token punctuation">(</span>Dep<span class="token punctuation">.</span>target<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// Dep.target也就是Watcher实例  </span>
      <span class="token keyword">return</span> val<span class="token punctuation">;</span>  
    <span class="token punctuation">}</span><span class="token punctuation">,</span>  
    <span class="token function">set</span><span class="token punctuation">(</span>newVal<span class="token punctuation">)</span> <span class="token punctuation">{</span>  
      <span class="token keyword">if</span> <span class="token punctuation">(</span>newVal <span class="token operator">===</span> val<span class="token punctuation">)</span> <span class="token keyword">return</span><span class="token punctuation">;</span>  
      dep<span class="token punctuation">.</span><span class="token function">notify</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 通知dep执行更新方法  </span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>  
  <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">}</span>  
</code></pre></div><h4 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h4> <ul><li><a href="https://www.liaoxuefeng.com/wiki/1022910821149312/1109527162256416" target="_blank" rel="noopener noreferrer">https://www.liaoxuefeng.com/wiki/1022910821149312/1109527162256416<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li> <li><a href="https://juejin.cn/post/6844903942254510087#heading-9" target="_blank" rel="noopener noreferrer">https://juejin.cn/post/6844903942254510087#heading-9<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li> <li><a href="https://juejin.im/post/6844903586103558158#heading-11" target="_blank" rel="noopener noreferrer">不好意思！耽误你的十分钟，让MVVM原理还给你<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li> <li><a href="https://segmentfault.com/a/1190000006599500" target="_blank" rel="noopener noreferrer">剖析Vue原理&amp;实现双向绑定MVVM - SegmentFault 思否<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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